The present invention relates to a method for producing an alternating phase mask which is suitable for patterning semiconductor components.
Phase masks can be used to influence the phase relationships, in a targeted manner, of the light used for exposing photosensitive layers on semiconductor components. By using such a phase mask, the image contrast and the depth of focus are improved in the case of an optical projection exposure. By way of example, it is known from Nxc3x6lscher, VDI Reports 935 (1991), pages 61-80 that multistep phase masks can be used as a transition between two regions with a different phase shift in order to avoid an expensive double exposure (trim exposure) of a semiconductor substrate.
Furthermore, it is known from the prior art that e.g. photoresist can be used as a phase shifter. This is described, for example, in Watanabe et al., SPIE 1463 (1991), page 110 et seq., and Miyazaki et al., SPIE 1464 (1991), page 327 et. seq. However, these methods are unsuitable for practical use with an exposure wavelength in the deep UV (Ultraviolet) range.
A single-step resist thickness modulation is used by Kobayashi et al., SPIE 3873 (1999), pages 288-296. The different resist thickness is utilized during dry etching in order to pattern half-tone phase masks with a chromium mask layer. In contrast thereto, in the present application, a half-tone phase mask is not produced, but rather an alternating phase mask is produced and the carrier material is etched.
Patterning a multistep transition between regions of different phases is described in Issued European Patent EP 0 451 307 B1, for example. What is disadvantageous in this case, however, is that a complete process sequence comprising resist coating, exposure, development, and etching of the quartz carrier has to be carried out for each region of the phase mask that is produced for a defined phase shift. Consequently, using multistep phase masks is expensive and impracticable for defect-free masks.
German Patent Application DE 197 56 518 A1 and Issued German Patent DE 42 29 157 C2 show phase masks with a stepped transition between the phase shifter and the substrate. Etching masks that are progressively recessed are used to produce the stepped transition.
It is accordingly an object of the invention to provide a method for producing an alternating phase mask which overcomes the above-mentioned disadvantages of the prior art methods of this general type.
In particular, it is an object of the invention to provide a method which enables a phase mask to be produced cost-effectively and in a manner that saves time.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing an alternating phase mask that includes steps of: providing a carrier having a surface with a mask layer configured thereon; applying a first irradiation-sensitive layer on the mask layer; irradiating the first irradiation-sensitive layer; developing the first irradiation-sensitive layer and partly removing the first irradiation-sensitive layer to form a first exposure structure; using the first exposure structure as an etching mask while etching the mask layer to remove the mask layer at locations where the first irradiation-sensitive layer was removed; after etching the mask layer, removing the first exposure structure; applying a second irradiation-sensitive layer to the mask layer and the carrier; determining a first exposure dose and a second exposure dose; at locations at which the first mask layer has been removed, irradiating the second irradiation-sensitive layer with the first exposure dose and elsewhere with the second exposure dose; developing the second irradiation-sensitive layer to form a second exposure structure that has a first exposure structure thickness at locations that were exposed with the first exposure dose and that has a second exposure structure thickness, different from the first exposure structure thickness, at locations that were exposed with the second exposure dose; and etching the carrier at locations that are not covered by the second exposure structure and etching the second exposure structure with essentially the same etching selectivity such that the carrier is etched down to a first etching depth in the region of the first exposure structure thickness and down to a second, smaller etching depth in the region of the second exposure structure thickness.
The second exposure structure is exposed by multiple irradiations with different doses. This can be performed for example, using light, electrons or ions. After conventionally developing the second irradiation-sensitive layer, a second exposure structure having a first thickness and a second thickness is formed in the second irradiation-sensitive layer. An etching step, which can be performed, for example, as an isotropic or as an anisotropic etching step, transfers the exposure structure to the carrier. By way of example, if the second exposure structure and the carrier have a similar etching selectivity and if an anisotropic etching is performed, then the step structure of the second irradiation-sensitive layer with the different exposure structure thicknesses is transferred to the carrier. Afterwards, the second exposure structure can be removed from the carrier. The phase mask produced is advantageously robust with respect to cleaning steps. Furthermore it can be produced with significantly fewer process steps than is known from the prior art. The second exposure structure is exposed by irradiation with short-wave light, electrons or ions, in particular by a laser beam or an electron beam. The laser or electron beam is guided over the area to be exposed using a corresponding control. In this case, according to the invention, the exposure dose is modulated. Irrespective of whether laser beams or electron beams are used, the term exposure is used in both cases.
In accordance with an added feature of the invention, the carrier is etched isotropically while undercutting the mask layer or the second exposure structure parallel to the surface. So-called intensity balancing is advantageously made possible by the undercut. This is known for example from the publication Griessinger et al., SPIE 3873 (1999), pages 359-369. The intensity balancing performed has the advantage that the imaged intensity becomes independent of the etching depth. The intensity difference in the image of adjacent mask openings becomes largely independent of the etching depth of the openings.
In accordance with an additional feature of the invention, before etching the carrier, the second exposure structure is heated to a temperature of between 120xc2x0 C. and 190xc2x0 C. such that a stepxe2x80x94arranged between the first exposure structure thickness and the second exposure structure thicknessxe2x80x94in the second exposure structure becomes rounded.
The rounding or flowing of the second exposure structure has the advantage that a continuous and smooth transition, similar to a ramp, is produced in the second exposure structure. During the subsequent etching of the carrier, the continuous ramp is transferred to the carrier, thereby producing a continuous transition in the phase mask between a first region with a first phase shift and a second region with a second phase shift.
In accordance with a further feature of the invention, the carrier is formed from quartz and/or the mask layer is formed from chromium.
In accordance with a concomitant feature of the invention, the etching of the carrier is performed using an etching gas that contains CHF3 and oxygen. A mixture of CHF3 and oxygen makes it possible, for example, to set an etching selectivity with which the second exposure structure is etched at a similar etching rate to the carrier.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing an alternating phase mask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.